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Abstract
Mammalian mitochondrial DNA (mtDNA) encodes 13 proteins that are essential for the function of the oxidative phosphorylation system, which is composed of four respiratory-chain complexes and adenosine triphosphate (ATP) synthase. Remarkably, the maintenance and expression of mtDNA depend on the mitochondrial import of hundreds of nuclear-encoded proteins that control genome maintenance, replication, transcription, RNA maturation, and mitochondrial translation. The importance of this complex regulatory system is underscored by the identification of numerous mutations of nuclear genes that impair mtDNA maintenance and expression at different levels, causing human mitochondrial diseases with pleiotropic clinical manifestations. The basic scientific understanding of the mechanisms controlling mtDNA function has progressed considerably during the past few years, thanks to advances in biochemistry, genetics, and structural biology. The challenges for the future will be to understand how mtDNA maintenance and expression are regulated and to what extent direct intramitochondrial cross talk between different processes, such as transcription and translation, is important.
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Affiliation(s)
- Claes M Gustafsson
- Department of Medical Biochemistry and Cell Biology, University of Gothenburg, 405 30 Gothenburg, Sweden; ,
| | - Maria Falkenberg
- Department of Medical Biochemistry and Cell Biology, University of Gothenburg, 405 30 Gothenburg, Sweden; ,
| | - Nils-Göran Larsson
- Department of Mitochondrial Biology, Max Planck Institute for Biology of Ageing, 50931 Cologne, Germany; .,Department of Medical Biochemistry and Biophysics, Karolinska Institutet, 171 77 Stockholm, Sweden
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St. John JC, Facucho-Oliveira J, Jiang Y, Kelly R, Salah R. Mitochondrial DNA transmission, replication and inheritance: a journey from the gamete through the embryo and into offspring and embryonic stem cells. Hum Reprod Update 2010; 16:488-509. [DOI: 10.1093/humupd/dmq002] [Citation(s) in RCA: 196] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
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Falkenberg M, Larsson NG, Gustafsson CM. DNA replication and transcription in mammalian mitochondria. Annu Rev Biochem 2007; 76:679-99. [PMID: 17408359 DOI: 10.1146/annurev.biochem.76.060305.152028] [Citation(s) in RCA: 479] [Impact Index Per Article: 28.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The mitochondrion was originally a free-living prokaryotic organism, which explains the presence of a compact mammalian mitochondrial DNA (mtDNA) in contemporary mammalian cells. The genome encodes for key subunits of the electron transport chain and RNA components needed for mitochondrial translation. Nuclear genes encode the enzyme systems responsible for mtDNA replication and transcription. Several of the key components of these systems are related to proteins replicating and transcribing DNA in bacteriophages. This observation has led to the proposition that some genes required for DNA replication and transcription were acquired together from a phage early in the evolution of the eukaryotic cell, already at the time of the mitochondrial endosymbiosis. Recent years have seen a rapid development in our molecular understanding of these machineries, but many aspects still remain unknown.
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Affiliation(s)
- Maria Falkenberg
- Division of Metabolic Diseases, Karolinska Institutet, Novum, SE-141 86 Stockholm.
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Jucá MB, Aoyama H. Inhibition of poly(2'-fluoro-2'-deoxyadenylic acid)-directed-reverse transcriptase activity. JOURNAL OF ENZYME INHIBITION 1995; 9:171-7. [PMID: 8583254 DOI: 10.3109/14756369509042816] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Some intercalating and nonintercalating drugs have been tested as inhibitors on the DNA synthesis reaction catalyzed by avian myeloblastosis virus (AMV) reverse transcriptase, in the presence of polyriboadenylic acid (poly(rA)) and poly(2'-fluoro-2'-deoxyadenylic acid) (poly(dAfl)) as templates. In both cases, the inhibition was higher with the intercalating drug ethidium bromide than with the nonintercalating analog tetramethyl ethidium bromide. Ethidium bromide inhibited more efficiently the poly(rA)- than the poly(dAfl)-directed reverse transcriptase reaction; in the latter case, the inhibition was non-competitive in relation to TTP. On the other hand, the reaction catalyzed in the presence of the 2'-fluorinated polynucleotide as template was inhibited to a higher extent by other nonintercalating drugs, berenil, netropsin, and distamycin. The inhibitions of both reactions by dideoxy TTP, novobiocin and HPA-23 are also discussed.
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Affiliation(s)
- M B Jucá
- Departamento de Bioquimica, Universidade Estadual de Campinas, São Paulo, Brasil
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Hart GJ, Orr DC, Penn CR, Figueiredo HT, Gray NM, Boehme RE, Cameron JM. Effects of (-)-2'-deoxy-3'-thiacytidine (3TC) 5'-triphosphate on human immunodeficiency virus reverse transcriptase and mammalian DNA polymerases alpha, beta, and gamma. Antimicrob Agents Chemother 1992; 36:1688-94. [PMID: 1384425 PMCID: PMC192031 DOI: 10.1128/aac.36.8.1688] [Citation(s) in RCA: 126] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
(-)-2'-Deoxy-3'-thiacytidine (3TC) is a selective inhibitor of human immunodeficiency virus replication in vitro (J. A. V. Coates, N. Cammack, H. J. Jenkinson, A. J. Jowett, M. I. Jowett, B. A. Pearson, C. R. Penn, P. L. Rouse, K. C. Viner, and J. M. Cameron, Antimicrob. Agents Chemother. 36:733-739, 1992). The effect of 3TC 5'-triphosphate on both the RNA-dependent and DNA-dependent activities of human immunodeficiency virus type 1 reverse transcriptase and DNA polymerases alpha, beta, and gamma from HeLa cells was investigated. 3TC 5'-triphosphate is a competitive inhibitor (with respect to dCTP) of the RNA-dependent DNA polymerase activity (apparent Ki = 10.6 +/- 1.0 to 1.24 +/- 5.1 microM, depending on the template and primer used); the DNA-dependent DNA polymerase activity is 50% inhibited by a 3TC 5'-triphosphate concentration of 23.4 +/- 2.5 microM when dCTP is present at a concentration equal to its Km value. Chain elongation studies show that 3TC 5'-triphosphate is incorporated into newly synthesized DNA and that transcription is terminated in a manner identical to that found for ddCTP. The 50% inhibitory concentrations of 3TC 5'-triphosphate against DNA polymerases alpha, beta, and gamma at concentrations of dCTP equal to the Km were 175 +/- 31, 24.8 +/- 10.9, and 43.8 +/- 16.4 microM, respectively. More detailed kinetic studies with 3TC 5'-triphosphate and DNA polymerases beta and gamma are consistent with the fact that inhibition of these enzymes by 3TC 5'-triphosphate is competitive with respect to dCTP. The values of Ki were determined to be 18.7 microM for DNA polymerase beta and 15.8 +/- 0.8 microM for DNA polymerase gamma.
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Affiliation(s)
- G J Hart
- Department of Virology, Glaxo Group Research Ltd., Greenford, United Kingdom
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Katinka MD. RNA-dependent DNA polymerase activity in Paramecium tetraurelia: what for? EUROPEAN JOURNAL OF BIOCHEMISTRY 1987; 163:569-75. [PMID: 2435548 DOI: 10.1111/j.1432-1033.1987.tb10905.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Protein extracts from the protozoan ciliate Paramecium tetraurelia revealed high levels of RNA-dependent DNA polymerase activity (reverse transcriptase). Stable and constant during the somatic phase of the cell cycle, the reverse transcriptase activity quickly diminished following the completion of the sexual phases of the cell cycle: conjugation and autogamy. The Paramecium reverse transcriptase presented a number of common features with retroviral polymerases: ability to copy synthetic templates such as poly(rCm).oligo(dG) as well as mRNA; sensitivity to various reverse transcriptase inhibitors such as HPA 23, suramin, phosphonoformate and ethidium bromide; insensitivity to the action of other DNA and RNA polymerase inhibitors and, finally, the requirement for divalent cations before the enzyme can function: either magnesium or manganese. Although the reverse transcriptase activity was not proven to be independent from one of the DNA polymerases in paramecia, its high activity predicts a role in the paramecia cell cycle. From what we are able to conceive today two possible roles could be envisaged. Participation in the anlage macronucleus formation: micronuclear sequences are first transcripted and, after rearrangements of the RNA molecules, these are retrotranscribed into the macronuclear DNA molecules or association with retrotransposons that participate in the movement of certain macronuclear sequences into the germ-line micronucleus.
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Benzair A, Rhodes-Feuillette A, Emanoil-Ravier R, Périès J. Inhibition by ethidium bromide of simian foamy virus type 1 RNA-dependent DNA polymerase. ACTA ACUST UNITED AC 1985. [DOI: 10.1016/s0769-2617(85)80116-7] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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Seki S, Oda T. Differential sensitivity to ethidium bromide of replicative DNA synthesis and bleomycin-induced unscheduled DNA synthesis in permeable mouse sarcoma cells. EXPERIENTIA 1984; 40:869-71. [PMID: 6205893 DOI: 10.1007/bf01952001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Replicative DNA synthesis in permeable mouse sarcoma cells was more sensitive to ethidium bromide (EtBr) than bleomycin-induced unscheduled DNA synthesis (UDS). A similar difference in sensitivity to EtBr was observed between DNA polymerases alpha and beta. The difference in sensitivity to EtBr of replicative DNA synthesis and UDS in the present system seems to reflect mainly the sensitivity difference between DNA polymerases alpha and beta.
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Benzair AB, Rhodes-Feuillette A, Emanoil-Ravicovitch R, Peries J. Characterization of RNase H activity associated with reverse transcriptase in simian foamy virus type 1. J Virol 1983; 47:249-52. [PMID: 6191042 PMCID: PMC255244 DOI: 10.1128/jvi.47.1.249-252.1983] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Spumavirinae or foamy viruses have been shown to have a characteristic RNA-dependent DNA polymerase activity. We demonstrate here the existence of an RNase H activity that copurifies with the 81-kilodalton monomeric polypeptide, which carries the RNA-dependent DNA polymerase activity of simian foamy virus type 1. RNase H degrades RNA hybrid substrates; however, it does not solubilize single-stranded RNAs. Inactivation assays with heat, high levels of bivalent cations, ethidium bromide, and sodium fluoride suggest that the RNase H catalytic site could be topologically independent from the DNA polymerase catalytic site.
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Sarih L, Hevia-Campos E, Tharaud D, Litvak S. Inhibition of avian myeloblastosis virus reverse transcriptase by ethidium bromide: effect on enzyme activity and primer tRNA recognition. FEBS Lett 1980; 122:100-4. [PMID: 6163656 DOI: 10.1016/0014-5793(80)80411-x] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
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Lidak MY, Paégle RA. Problems in the search for new antiblastomogenic substances. Pharm Chem J 1977. [DOI: 10.1007/bf01152236] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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Chandra P, Ebener U, Steel LK, Laube H, Gericke D, Mildner B, Bardos TJ, Ho YK, Götz A. Molecular approaches to inhibit oncogenesis by RNA tumor viruses. Ann N Y Acad Sci 1977; 284:444-62. [PMID: 280144 DOI: 10.1111/j.1749-6632.1977.tb21981.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
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Scott AC, Wells JR. Reiteration frequency of the gene for tissue-specific histone H5 in the chicken genome. Nature 1976; 259:635-8. [PMID: 1250413 DOI: 10.1038/259635a0] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Chicken erythroid cells contain a tissue specific histone known as H5 in addition to the five major histone species found in other organisms. The mRNA coding for this histone has been isolated by indirect immunoprecipitation from immature, non-dividing reticulocytes in which this is the only histone synthesised. The mRNA has been modified by the enzymatic addition of a 3' polyadenylic acid tract, and transcribed into complementary DNA (cDNA) using the RNA-dependent DNA-polymerase from avian myeloblastosis virus. Studies on the hybridisation of this cDNA indicate that the gene coding for the H5 histone is reiterated 10 times in the chicken genome.
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Zunino F, Gambetta R, Colombo A, Luoni G, Zaccara A. DNA polymerases of rat liver. Partial characterization and effect of various inhibitors. EUROPEAN JOURNAL OF BIOCHEMISTRY 1975; 60:495-504. [PMID: 1204652 DOI: 10.1111/j.1432-1033.1975.tb21028.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Three distinct DNA-dependent DNA polymerase activities have been partially purified from normal rat liver. Soluble activities are separable into two distinct fractions (P1 and P2) by phosphocellulose chromatography. A low-molecular-weight DNA polymerase was isolated from purified nuclei. The enzymes were characterized according to chromatographic and sedimentation behavior, enzymological properties, and response to various inhibitors. The results indicate that fraction P1 corresponds to the high-molecular-weight enzyme and suggest that polymerase P2 may be derived from partial dissociation of the high-molecular-weight enzyme. The molecular weight of polymerase P1 was estimated to be about 250 000 by Sephadex column chromatography. Both fraction P2 and nuclear DNA polymerase appeared to be low-molecular-weight enzymes. However, the molecular size of these activities was apparently different. The estimated molecular weights of nuclear and P2 enzyme are about 40 000 and 25 000, respectively. As with the nuclear enzyme, polymerase P2 (but not P1) appeared to be free of detectable exonuclease activity. All of these polymerases showed a marked preference for initiated polydeoxyribonucleotide templates. The rat liver polymerases differed in their ability to use poly[d(A-T)-A1 primer-template, as is shown by the ratios of their activity with this synthetic polymer to that with activated DNA: 0.5, 2.75, and 1.34 for P1, P2, and nuclear polymerase, respectively. Denatured DNA was a poor template for both enzymes P1 and P2, but it was inert as template for the nuclear enzyme. Although each of these polymerases required all four deoxynucleoside triphosphates for maximal activity, they catalyzed a high rate of synthesis in the absence of one or more deoxynucleoside triphosphates. Such a 'limited' synthesis was much more extensive for polymerase P2 and nuclear enzyme than for P1 was the most sensitive of the three to sulphydryl reagents, ehtidium bromide, heparin, and single-stranded DNA. The responses of P2 and nuclear enzymes to various inhibitors were very similar. However, these two enzymes respond differently to heat and high ionic strength.
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Roa RC, Bose SK. Requirement for cellular protein synthesis in reversal of ethidium-bormide-induced inhibition of cell transformation by murine sarcoma virus. Proc Natl Acad Sci U S A 1975; 72:4337-40. [PMID: 172904 PMCID: PMC388716 DOI: 10.1073/pnas.72.11.4337] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Cultures of mouse Balb 3T3 fibroblasts exposed to a noncytotoxic dose of ethidium bromide for 16-18 hr are unable to produce foci after infection with murine sarcoma virus. Such cultures regain susceptibility to infection when incubated for 6-8 hr in drug-free growth medium. Pretreated but not untreated cultures exhibit sensitivity toward brief (6 hr) exposure to cycloheximide, chloramphenicol, and actinomycin D before infection. Pretreatment with cordy-cepin inhibits the ability of cultures to produce foci after infection. The recovery of ethidium-bromide-treated cultures requires the synthesis of cellular proteins which may have some important role in the establishment of RNA tumor virus infection.
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Guntaka RV, Mahy BW, Bishop JM, Varmus HE. Ethidium bromide inhibits appearance of closed circular viral DNA and integration of virus-specific DNA in duck cells infected by avian sarcoma virus. Nature 1975; 253:507-11. [PMID: 163973 DOI: 10.1038/253507a0] [Citation(s) in RCA: 115] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
The DNA of avian sarcoma virus assumes a closed circular configuration before integration into the host cell chromosomal DNA. Ethidium bromide reduces the formation of superhelical viral DNA and concurrently blocks integration of the viral genome. Inhibition of integration of viral DNA results in the inhibition of virus replication.
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RNA-Directed DNA Polymerase—Properties and Functions in Oncogenic RNA Viruses and Cells1. ACTA ACUST UNITED AC 1974. [DOI: 10.1016/s0079-6603(08)60208-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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Chandra P, Di Marco A, Zunino F, Casazza AM, Gericke D, Giuliani F, Soranzo C, Thorbeck R, Götz A, Arcamone F, Ghione M. Influence of some antitumor antibiotics on viral neoplasia. The role of molecular structure in the inhibition of DNA polymerases from RNA tumor viruses, viral multiplication and tumor growth by some antitumor antibiotics. THE SCIENCE OF NATURE - NATURWISSENSCHAFTEN 1972; 59:448-55. [PMID: 4117558 DOI: 10.1007/bf00592879] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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Citarella RV, Muller R, Schlabach A, Weissbach A. Studies on vaccinia virus-directed deoxyribonucleic acid polymerase. J Virol 1972; 10:721-9. [PMID: 4673490 PMCID: PMC356526 DOI: 10.1128/jvi.10.4.721-729.1972] [Citation(s) in RCA: 45] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
A vaccinia-directed deoxyribonucleic acid (DNA) polymerase has been partially purified from the cytoplasmic fractions of virus-infected HeLa cells. The utilization of natural and synthetic templates by this enzyme resembles that of the host cell DNA-dependent DNA polymerases. The vaccinia DNA polymerase cannot copy ribopolymers or ribonucleic acid but is very effective with an "activated" DNA as template. An exonuclease preferring single-stranded DNA as substrate is found in the most highly purified preparations of the enzyme. The molecular weight of the vaccinia DNA polymerase seems to be about 110,000. The viral DNA polymerase is also found to be associated with purified, infected cell nuclei, and this association may be due, at least in part, to nonspecific adsorption of the vaccinia DNA polymerase by nuclei.
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Chepelinsky AB, Bell E. I-DNA suppressed by ethidium bromide and accumulated with dactinomycin. NATURE: NEW BIOLOGY 1972; 239:44-7. [PMID: 4507212 DOI: 10.1038/newbio239044a0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
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Chandra P, Zunino F, Götz A. Bis-deae-fluorenone: A specific inhibitor of DNA polymerases from RNA tumor viruses. FEBS Lett 1972; 22:161-164. [PMID: 11946586 DOI: 10.1016/0014-5793(72)80034-6] [Citation(s) in RCA: 39] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- P Chandra
- Institut für Therapeutische Biochemie der Universität Frankfurt/M., Germany
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Chandra P, Zunino F, Götz A, Gericke D, Thorbeck R, Di Marco A. Specific inhibition of DNA-polymerases from RNA tumor viruses by some new daunomycin derivatives. FEBS Lett 1972; 21:264-268. [PMID: 11946523 DOI: 10.1016/0014-5793(72)80179-0] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- P Chandra
- Institut für Therapeutische Biochemie der Universität, Frankfurt/M., W. Germany
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Chandra P, Zunino F, Götz A, Wacker A, Gericke D, Di Marco A, Casazza AM, Giuliani F. Template specific inhibition of DNA polymerases from RNA tumor viruses by distamycin A and its structural analogues. FEBS Lett 1972; 21:154-158. [PMID: 11946499 DOI: 10.1016/0014-5793(72)80126-1] [Citation(s) in RCA: 32] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Affiliation(s)
- P Chandra
- Institut für Therapeutische Biochemie der Universität, /M., Frankfurt, Germany
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Fridlender B, Fry M, Bolden A, Weissbach A. A new synthetic RNA-dependent DNA polymerase from human tissue culture cells (HeLa-fibroblast-synthetic oligonucleotides-template-purified enzymes). Proc Natl Acad Sci U S A 1972; 69:452-5. [PMID: 4501125 PMCID: PMC426478 DOI: 10.1073/pnas.69.2.452] [Citation(s) in RCA: 103] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
Two DNA polymerases that can copy synthetic RNA polymers are present in human tissue culture cells. These enzymes which have each been purified about 500-fold, are present in both HeLa cells, which are derived from a cervical carcinoma, and in WI-38 cells, a normal diploid strain originating from human embryonic lung tissue. These synthetic RNA-dependent DNA polymerases are identified by their ability to copy efficiently the ribo strand of synthetic oligonucleotide-homopolymer complexes, and differ in this respect from the known DNA-dependent DNA polymerases found in HeLa cells. The template requirements of these new DNA polymerases resemble that of the RNA-dependent DNA polymerases of the RNA tumor-viruses.
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